This invention relates to a cascade aerosol impactor for classifying aerosol particles that includes pressure sensors for monitoring the functions of the impactor, and further includes mounting structure that permits rotating impaction plates without mechanical drives that require seals. The nozzles used are also constructed to reduce cross flow effects.
Inertial impactors are widely used for measuring the size distribution of aerosols. For purpose of this invention, particles suspended in a gas are referred to as an aerosol. The gas is usually air, but other gases such as nitrogen, oxygen, argon, helium, etc. can also be the suspending gaseous medium. The particles can be a solid, a liquid, or a mixture of both. The particle size is usually between 0.002 xcexcm and 100 xcexcm.
Inertial impactor, aerosol impactor, or impactor all refer to an aerosol sampling or collection device that separates aerosol particles from the gaseous medium in which they are suspended by the inertial effect of the particles. The device usually uses a nozzle to accelerate the gas to a high velocity and direct the gas jet against an impaction plate to cause particle impaction on the plate. Particles will impact only when their size is larger than a certain critical value, while smaller particles with insufficient momentum or inertia will be carried by the gas flow around the plate to an exit and escape collection.
The critical particle size at which particle collection occurs is referred to as the impactor cut-point. The cut-point particle diameter of an impactor can be varied by varying the nozzle size and gas velocity. Smaller nozzles and higher gas velocities will produce smaller cut-point particle diameters. The impactor cut-point is also affected by the gas viscosity, the shape of the nozzle, as well as the nozzle-to-plate distance. In an ideal impactor all particles larger than the cut-point are collected with 100% efficiency while smaller particles are not collected. In a real impactor, particle impaction does not occur ideally at a single particle size. The transition from zero to 100% particle collection usually occurs over a range of particle sizes. The narrower this range, the sharper the impactor cut-size characteristics. The ideal impactor is then an impactor with a perfect cut value. In a real impactor with less than a perfect sharpness-of-cut, the cut-point is usually defined as the particle diameter at which 50% of the particles are collected.
The prior art impactor just described is a single stage impactor. It consists of a single nozzle plate carrying one or more nozzles in parallel and an adjacent impaction plate. Several single-stage impactors can be arranged in series to form a cascade impactor. Cascade impactors are designed so that large particles are collected first, followed by smaller and smaller particles between an inlet and an outlet. There is usually a final filter to collect small particles below the cut-point of the last impactor stage. For instance, in a three-stage impactor with cut-point diameters of, say 10, 3 and 1 xcexcm, particles larger than 10 xcexcm are removed by the first, 10-xcexcm cut stage. Subsequently, particles in the 3-to-10 xcexcm and the 1-to-3 xcexcm ranges are removed by the 3-xcexcm and 1-xcexcm cut stages, respectively. The final filter then collects particles smaller than 1 xcexcm. FIG. 1 is a schematic diagram of a prior art three-stage impactor with a final filter, in which a single nozzle of a progressively smaller diameter is used in each stage to increase the gas velocity to higher and higher values to collect smaller and smaller particles.
The cascade impactor is very useful for size distribution analysis of aerosol particles. Particulate air pollutants, aerosols in the work place environment, as well as other aerosols of practical interest are usually polydisperse, with particle sizes spread over a wide range of values. Cascade impactors can be used to separate particles by size into narrower intervals for analysis. The size-fractionated particles can then be analyzed gravimetrically to determine their mass size distribution. Alternatively, the particles can be analyzed chemically to determine the chemical composition of the particles as a function of particle size. Cascade impactors are widely used in air pollution studies to determine the physical and chemical properties of the airborne particles, assess their potentially harmful health effect, or determine the origin of the particles for pollution abatement or control purposes.
In the schematic prior art diagram of FIG. 1, an aerosol source 6 is connected to an inlet of a cascade impactor 8. Impactor 8 has a single nozzle 9A, 9B and 9C in each impactor stage 8A, 8B and 8C. Impaction plates 10A, 10B and 10C are provided and a filter 11 follows impactor stage 8C. An outlet 12 leads to a pump or a blower 13. In practical impactors, each impactor stage is usually comprised of a number of nozzles formed in a nozzle plate. The flow through each nozzle can be quite small, but the total flow through all the nozzles can be quite high by using a large number of nozzles in parallel. A high sampling flow rate is needed to increase sample collection so that the quantity of particulate matter collected is sufficient for analysis.
In recent years, demands for increased accuracy and precision for aerosol measurement have led to the development of cascade impactors with a high volumetric flow rate and large number of impactor stages. For instance, the Micro-Orifice Uniform Deposit Impactor, sold under the trademark MOUDI(trademark), manufactured by the MSP Corporation of Minneapolis, Minn., the assignee of this application, comprises eight (8) or ten (10) impactor stages with nominal cut-point particle diameters that range between 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1 and 0.056 xcexcm. In the final stages, nozzle diameters as small as 50 xcexcm are used. To provide the needed 30 liter-per-minute sampling flow rate, as many as 2,000 nozzles are used in some stages.
The need for increased measurement sensitivity in air pollution research and for other applications has created the need for impactors with flow rates larger than the 30 liters-per-minute. To design impactors with higher flow rates, even larger number of nozzles need to be used. To create a cascade impactor with, say, 90 liters-per-minute sampling flow rate, and similar operational pressure drop characteristics, the number of small nozzles needs to be increased by a factor of 3. Thus, 6000 nozzles need to be used in the final stages of such high flow MOUDI(trademark) cascade impactors.
In designing impactors with large numbers of very small nozzles, it is important to consider the effect of cross flow in the impactor. As will be explained, when a nozzle plate with a large number of nozzles is provided, the gas flow through the outer nozzles, must pass radially outward across the surface of the nozzle plate between it and the impaction plate. This outward radial gas flow is referred to as the cross-flow. The cross flow can cause the gas jets through the nozzles in the cross flow path to be deflected side ways and change their cut-point characteristics. The sharpness-of-cut of the impactor as a whole will then decrease. The cross flow effect is the greatest for nozzles located near the outer edges of the nozzle cluster. To obtain good sharpness-of-cut characteristics, it is important to consider the cross flow effect in designing high flow impactors.
The use of large number of very small nozzles also creates the practical issue of nozzle plugging during use. As aerosols are sampled by the impactor, some accumulation of particulate matter around the edge of the nozzle is unavoidable. Over time, enough material can accumulate to partially block the flow and cause the cut-point of the impactor to change. This effect, if not monitored, can lead to measurement errors. The nozzles can usually be cleaned, but cleaning, if done improperly, for instance, by using a high intensity ultrasonic cleaner, can cause damage to the nozzle plate and alter the size of the nozzles.
Another issue relating to high accuracy, high precision modern impactor is that the use of a large number of impactor stages can cause errors in the assembly of the impactors. In the MOUDI(trademark) system referred to above, eight and ten impactor stages are used. Usually, the impactor stages must be assembled with impactor stage cut-points in a decreasing order. If, due to an operator mistake, the impactor stages are assembled incorrectly with the order of some stages reversed, erroneous data will be collected. The present invention solves the problems outlined, including the reduction of cross flow, monitoring changes in cut point and automatically detecting errors in the assembly and use of cascade impactors.
The present invention relates to a cascade impactor that provides a very accurate particle size cutoff at each of the stages of the impactor. The particle sizes are obtained by precise controlling of cross flow, and also by monitoring the performance of the nozzle plate. Additionally, enhanced operation is achieved by rotating the impaction plates utilizing a drive, which avoids the need for rotating seals between individual stages of the impactor. By monitoring the pressure drop across the individual stages and comparing it with standard or reference values of pressure drop obtained for similar nozzle plates, or obtained by calibrating the nozzle plates, any changes that indicate leaks, damage to the nozzle plates, or other detrimental factors can be determined quickly. Further, monitoring pressure also insures a check that the assembly of the various stages of the impactor is appropriate, because errors in assembly will cause changes in the pressure drop across the nozzle plates at different stages. Preset limits of pressure drop change can be set so that when the different pressure changes reach the set amount, alarms or other indicators can be activated for alerting an operator to a problem.
In one form of the invention, an optical sensor is used for providing signals indicating amount s of an aerosol provided by a drug delivery device on order to provide automatic measurement of such aerosol.
The overall construction thus insures simple operation, simple assembly and reliable, accurate classification of particles at quite xe2x80x9csharpxe2x80x9d cutoff points.
For purpose of this invention, the various terms are defined as follows:
Aerosol: Aerosol means small particles suspended in a gas. The gas is usually air, but can also be nitrogen, oxygen, argon, and other types of gases. The particles can be solid, liquid or a mixture of both. The particle size is usually between 0.002 xcexcm to 100 xcexcm.
Impactor: Impactor, inertial impactor, and aerosol impactor all refer to devices that collect aerosol particles by virtue of the inertial effect of particles. Impactors use a nozzle to accelerate the gas flow to a high velocity. The gas jet is then directed against an impaction plate to collect the suspended particle by inertial impaction. More than one such nozzle can be formed in a nozzle plate to increase the volumetric gas flow through the device.
Nozzle and orifice: A nozzle can be a flow passage with a converging cross section to accelerate the gas flow to a high velocity. It can also be a converging-diverging nozzle, such as a Venturi. A sharp-edge orifice and other forms of flow restrictions can also be used. The cross-sectional shape of the nozzle can be circular, rectangular, or a long slit of a large length-to-width ratio. Nozzles and orifices are used interchangeably in the present invention and may have any cross-sectional shape, perpendicular to the axis or plane of flow including, but not be limited to, that of a circle.
Nozzle throat: When the nozzle has a varying cross-section, the smallest cross-section is referred to as the throat. In the case of a nozzle with a circular cross-section, the diameter of the throat is the diameter of the smallest circular cross-section in the nozzle.
Aerodynamic equivalent diameter: An aerodynamic equivalent diameter of an aerosol particle of an arbitrary size, shape, and density is the diameter of a unit density sphere having the same settling velocity as the particle in question. The cut-point diameter of an impactor referred to in this description is based on the aerodynamic equivalent diameter of the particles.
Cascade impactor: A cascade impactor is a series of impactors formed in a single device. Each impactor in the series is then referred to as an impactor stage. The individual impactor stages are usually arranged in a descending order in stage cut-point. The particles collected by the individual stages can be weighed to determine the mass size distribution, or analyzed chemically to determine their chemical composition as a function particle size.
Computer: Computer is an electronic device for computing and data processing. It includes the popular, general-purpose Personal Computers (PC), and special-purpose electronic devices, such as the microprocessor, the microcontroller, and the like, that are used for data acquisition and processing, and special control functions.
Optical Aerosol Sensor: Optical aerosol sensor is an aerosol sensing device based on the scattering or transmission of light in an aerosol cloud. It measures the concentration of particles in the cloud by the light it scatters or transmits. It includes a light source projecting a beam of light through the cloud, a window or lens system to allow the scattered or transmitted light to fall onto a light sensor. The light source is usually a laser, an incandescent lamp, or a discharge lamp. The light sensor can be a solid state light detector commonly referred to as a photodiode, a photomultiplier tube, or a charge-coupled-device (CCD) of the type commonly used in still or video cameras. The wavelength of light is not limited to the 0.4 xcexcm of 0.7 xcexcm range that can be seen by the human eye, but include electromagnetic radiation in the range of the infrared, i.e.  greater than 0.7 xcexcm in wavelength, or the ultraviolet, i.e.  greater than 0.4 xcexcm in wavelength.